1. Field of the Invention
The present invention relates to a laser power monitor circuit to monitor the power of a laser used in a magneto-optical disk device and other related devices.
2. Description of the Related Art
The thermal properties of a laser beam are generally used in the field of magneto-optical recording. The recording and playback of a magneto-optical disk are performed using a light (laser) beam emitted from a laser. The light beam has a diameter of about 1 .mu.m, and is irradiated on the vertical magnetization film surface (recording surface) of the magneto-optical disk.
Recording on the magneto-optical disk is performed by a method known as Curie point writing. Laser light, intensity modulated according to the information to be recorded, is irradiated onto the magneto-optical disk. A spot of light is formed on the portion of the magneto-optical disk surface onto which the laser light is irradiated. Then, when the temperature of the recording film of the magneto-optical disk exceeds (becomes high) the Curie point (a temperature fixed for the material used in the magneto-optical disk), this portion of the recording film becomes nonmagnetic. During the heating by laser light, an external magnetic field is impressed on the magneto-optical disk. When the portion which the light spot has irradiated moves from the position of the light spot as the magneto-optical disk rotates, the temperature of this portion of the recording film falls. At the point in time at which the temperature of the recording film falls below the Curie point temperature, the recording film is magnetized in the direction of the impressed external magnetic field. In this manner, recording is performed by changing the direction of magnetization.
In the optical recording medium of optical disks and magneto-optical disks and the like, tracks used to record information are formed in a vortex form or as concentric circles. One or many of these tracks are formed per revolution. Two kinds of information units, corresponding to 0 and 1, are formed on these tracks and information is recorded.
Generally, the magnetization of the tracks of a magneto-optical disk is made uniform in one direction (for example, upwards) by impressing a strong external magnetic field before recording. After this, marks having the reverse direction of magnetization (for example, downwards) are formed on the tracks. Information is represented by these marks, for example, by their presence or absence, position of the front end (front edge) of the mark, position of the rear end (rear edge) of the mark, length of the mark, etc. In particular, the method in which the position of the edges of the mark represents information is called mark length recording. In the past, these marks were called pits or bits, but have recently been referred to as marks.
Such marks are formed using heat, as mentioned above. Changes in the laser power at this time become as shown in FIG. 14. The level of the laser power when a mark is formed is the write power, Pw, and the level of the laser power when a mark is not being formed is the pedestal power, Pped.
However, a mark is not formed if the temperature of the recording layer fails to exceed the Curie point. That is, even if the power level of the laser is the write power, if the temperature of the recording film does not become higher than the Curie point, a mark is not formed. Because of this, since the temperature of the recording film does not immediately reach the Curie point in the front edge portion of the mark, the mark is slow to form. The shape of the front edge portion of the mark becomes narrow and short. Moreover, on the other hand, in the rear edge portion of the mark, because the temperature of the recording film does not immediately fall below the Curie point, the shape of the mark does not immediately end. Therefore, the shape of the mark in the rear edge portion becomes thick and long. Because of this, the mark cannot be formed in the appropriate shape.
Consequently, by causing the laser power to change as in FIG. 15, a recording correction is performed by edge intensity adjustment. In the portion to form the front edge of the mark, the laser power is made higher than the write power, and in the portion to form the rear edge of the mark, the power is made lower than the pedestal power. A mark of appropriate shape is made by proceeding in this manner. At this time, the amount of laser power at the time of intensity adjustment of the front edge of the mark (the power of light generated which is greater than the write power) is called an overshoot amount, and the amount of laser power at the time of intensity adjustment of the rear edge of the mark (the power of light generated which is smaller than the pedestal power) is called the undershoot amount. The overshoot amount and undershoot amount are collectively termed "recording correction amounts". These recording correction amounts are fixed as the edge intensity adjustment amounts I1 and I2, respectively, and have a time constant .tau..
Moreover, when recording data, the laser power level is controlled, and the laser power is monitored so that recording is performed at the appropriate laser power. Nevertheless, even when controlling the laser power level by monitoring the laser power as mentioned above, when the monitoring of laser power is performed at the time of performing a recording correction, there are problems in accurately controlling the power of the laser light. The following occurs as a result.
When monitoring the power of the laser at the time of performing the recording correction of a portion of the front edge of a mark (namely, before reducing the laser power down to the write level), because the laser power is higher than the write level, the result of performing the control is that the write power level of the laser becomes lower than the write power Pw. Similarly, when monitoring the laser power at the time of performing recording correction of a portion of the rear end of a mark, the end result is higher than the pedestal power Pped.